Welcome, Guest: Login or Register My Cart (0)

Frequently Asked Questions

Here we have provided comprehensive lists of commonly asked questions, process and procedures —please find questions and answers that are specific to Optical Filters.

Angle Tuning

AOI effect on RazorEdge spectraThe spectrum of any thin-film filter shifts toward shorter wavelengths when the angle of incidence (AOI) of light upon the filter is increased from 0° (normal incidence) to larger angles. In most cases, the filter spectrum becomes highly distorted at larger angles, and the shift can be significantly different for s- and p-polarized light. The spectral shift that you experience is highly dependent upon filter design. To learn more about how angle tuning affects the spectral performance of different Semrock filters, please review our technical note: Filter Spectra at Non-normal Angles of Incidence.

VersaChrome® Tunable Bandpass Filters

VersaChrome filters are a revolutionary new optical filter technology: thin-film filters that are tunable over a wide range of wavelengths by adjusting the angle of incidence with essentially no change in spectral performance. More information about tunable filters can be found in our technical library.

If you have further questions about a specific application, please contact technical support for assistance.

MyLight™

Try angle tuning yourself, by using MyLight™. Select your filter and click on the blue "MyLight" button above the spectra to model the theory data at whatever angle you desire.

AOI and CHA

Angle of Incidence, or AOI, is the angle at which a collimated beam of light is incident on the filter's first surface, measured relative to the surface normal. Cone half-angle, or CHA, refers to the angular range associated with a non-collimated incident beam, and is measured from the AOI to the largest cone angle.


Angle of Incidence and Cone Half-angle

 

Changing the AOI or CHA of your incident light will change the spectral response of the filter. For more information on how different types of Semrock filters respond to changes in AOI, please review our technical note: Filter Spectra at Non-normal Angles of Incidence. The spectral response to CHA is dependent upon many factors, so please contact technical support to discuss your specific application.

The AOI and CHA ranges for which filter performance is guaranteed are listed in each filter's Specifications tab on the website. The AOI and CHA specifications given are exclusive; filters may not meet specification when incident light has non-zero value for CHA and AOI.

ASCII Data

ASCII data is typically a text file with two columns. The first column is the wavelength (measured in nanometers) and the second column is the corresponding transmission value of the filter at that wavelength. The transmission value is either on a scale of 0 to 1 or of 0% to 100%. ASCII data is available to download for all filters on the Semrock website. Most filters show actual measured data, while a few filters have theoretical data listed. The legend indicates that the displayed data is either measured or theoretical.

When you are viewing a filter's details you will find a link to the ASCII data for that filter to the right of the graph. Click on ASCII Data to save a copy of the text file that can be used to graph the filter's spectra.

 

 

If you need theory data on a single filter, use the blue MyLight button found above the graph and model your theory data, as well as download your ASCII data.

Bandwidth

Semrock uses a “manufacturable specification” approach to define the bandwidth of our BrightLine® bandpass filters. We believe this approach more accurately reflects the performance of the filter in an optical system.

As shown in the diagram, the filter spectrum (red line) must lie within the unshaded regions. The average transmission must exceed the specification Tavg (%) in the Transmission Region, which has a certain center wavelength (CWL) and a width called the Guaranteed Minimum Bandwidth (GMBW). The filter part number has the form FF01-{CWL}/{GMBW}.

The transmission must lie below the blocking level specifications (OD) in the Blocking Regions. The precise shape of the spectrum is unspecified in the Transition regions. However, typically the filter passband has a Full Width at Half Maximum (FWHM) that is about 1% of the CWL wider than the GMBW bandwidth,

or


So, for the example shown in the diagram, the FF01-520/35 filter has a GMBW of 35 nm and a FWHM of 35 nm + 1% of 520 nm, or 40 nm.

Clear Aperture

While the optical coating extends to the edge of every Semrock filter, the clear aperture will be less than the transverse dimensions for each filter.

For unhoused, round filters, the clear aperture is guaranteed to be greater than 85% of the outer diameter. For square and rectangular parts, the guaranteed clear aperture will be greater than 80% of the filter's transverse dimensions and is elliptically shaped. The clear aperture of a housed filter is dependent on the size and thickness of the housing ring. All filter specifications are guaranteed only within the clear aperture region.

Specific information can be found on the Specifications Tab for each individual filter.

Housed filter showing clear aperature example

Custom Coating

If you would like a quote on a custom filter, please fill out our Custom Filter Specification Form and email it to semrock@idexcorp.com.

Our high-volume, IBS sputtering technology provides unsurpassed performance, repeatability and value. Our designers will meet your optical filter needs and also add value in other areas, such as optical cell design and measurement platforms. From prototype through production, you can count on us for expert design advice, unsurpassed performance and rock-solid reliability. To learn more about how you can benefit from our experience, contact Semrock's technical support or call 866-SEMROCK to discuss all of the available options.

Custom Size a Filter

Semrock offers custom sizing of most catalog filters right on our website. Whether you need an unhoused / unmounted round or rectangular filter, or the filter mounted into one of Semrock’s standard-size aluminum housings, use our custom sizing tool to calculate the price for the quantity you require and part number and add it to your cart to purchase.

Most custom size filters are available in less than one week. Simply input your required diameter, rectangular dimensions or required housing diameter. Semrock can size to any diameter from 5 mm to 50 mm in whole 1 mm increments, along with the most common English sizes, 12.7 mm (1/2 inch), 25.4 mm (1 inch), and 50.8 mm (2 inches) in less than one week. Gotta have that 31.4mm unmounted diameter? We can accommodate that as well, our inside sales team will quote the lead time with your order acknowledgement (typically four weeks). Semrock also carries standard-size aluminum housings for the following dimensions: 12.5mm, 25mm, 25mm Sutter Threaded Rings, 32mm, and 50mm.

Each filter which is available for custom sizing lists the dimensional range which can be accommodated for that filter on the product page. The substrate thickness and tolerance will be the same as for the standard size part for the filter of interest, simply click on the Specifications tab for details.

Need a size outside of these limits, contact Semrock at Semrock@idexcorp.com or 1-866-Semrock to inquire.

Custom Sizing

Filter Effective Index 

What is the difference between the Semrock ‘Effective Index of Refraction’ neff also referred to as Filter Effective Index and the Index of Refraction n?

neff is a filter-specific value used to calculate the angle of incidence (AOI) dependent wavelength shift of spectral features. It is not related to the Index of Refraction n for the substrate that is coated to make the filter. For Fused Silica, n = 1.4570 at the specific wavelength of 632.8 nm. On the other hand, neff describes the entire filter, including the coatings on both sides of the filter as well as the substrate between those two coatings. Furthermore, neff differs from filter to filter, depending on filter design, even though all filters may be coated on the same substrate.

Filters have spectral features, e.g. a spectral edge at a particular wavelength that marks the transition between transmission and blocking. It is a characteristic of these filters that spectral features shift to lower wavelength with increasing AOI.

Because optical systems often generate light rays that are incident on filters at AOI > 0°, it is useful to be able to calculate how far the spectral features shift in wavelength with AOI. As described in the Technical Note Filter Spectra at Non-Normal Angles of Incidence, a spectral feature at wavelength λ0 shifts to wavelength λ at AOI = θ, according to a formula that contains neff:

Equation

Orientation of Semrock Filters 

Because of the durability of Semrock filters, you can easily populate filter cubes, filter sliders, and filter wheels yourself without fear of damaging the filters. To maximize intended transmission and blocking and to minimize autofluorescence, filters must always be oriented so that light is incident on a specific surface of the filter. This note describes the correct orientation for the different filter types.

Orienting Housed Excitation and Emission Filters

Semrock exciter and emitter filters mounted in housings feature an alignment arrow on the housing; see the illustrations below.  Orient such a filter so that the arrow points in the direction of light propagation. For microscopes, the exciter filter arrow should point away from the light source and toward the dichroic beamsplitter, and the emitter filter arrow should point away from the dichroic beamsplitter and toward the eye, detector, or camera.

Dichroic beamsplitters are rarely mounted in housings. See below for guidance.

 

Orienting Dichroic Beamsplitters and Other Unhoused Optical Filters

Dichroic beamsplitters and other unhoused optical filters feature orientation marks that identify the coated surface upon which light must be incident. An orientation mark is placed either on the front surface of the filter, or on the edge of the filter as a caret (^) mark. The different types of orientation marks are shown in the following drawings along with the corresponding orientation guidance.

  • Semrock logo: The logo is on the surface facing the incident light.
  • Line: A short line is on the surface facing the incident light. The line may be easier to see if viewed at an oblique angle.
  • Dot: A small dot is on the surface facing the incident light. The dot may be easier to see if viewed at an oblique angle.
  • Caret: A caret on the edge of the filter points in the direction of light travel. When the viewer faces the surface that receives the incident light, the caret points away from the viewer.

Caution: A number of dichroic beamsplitters have coatings on both surfaces. Always use the above instructions to identify the coated surface that should face the incident light! If you encounter any ambiguity or difficulty, please contact Semrock for assistance in identifying the surface orientation.

Further Resources

Complete how-to instructions for microscope cube assembly are available in PDF format and video.  

Understanding Filter, Substrate & Housing Thickness

 

 

 

 

 

Semrock offers two categories of individual catalog sized filters: unhoused filters such as dichroic beamsplitters, mirrors and tunable filters, or housed filters mounted in black anodized aluminum housings. Housed filters include bandpass filters, edge filters, and some dichroic beamsplitters.  For an unhoused filter, the unmounted filter thickness and unmounted substrate thickness are identical.  For a housed filter, the mounted filter thickness and unmounted substrate thickness are different, because the mounted filter thickness is the same as the housing thickness. 


Each housed filter is available in one standard filter (housing) thickness, either 3.5 mm or 5.0 mm. The 3.5 mm housed filters are typically used as emitters when part of a filter set; the 5.0 mm filters are most commonly used as exciters when part of a filter set.  The 3.5 mm thick housings can accommodate substrate thicknesses ranging from 1.05 mm to 2.0 mm; the 5.0 mm thick housings, from 1.05 mm to 3.5 mm.


The size of each filter mounted in a housing as a standard catalog configuration is shown on the Description and Pricing tab (see below) as “diameter x housing thickness”. 

 

 

Take for example the exciter and emitter filters from the GFP-3035D general-purpose filter set. The FF02-472/30-25 Exciter is mounted in a 5.0 mm aluminum housing, as shown below.

 

 

The FF01-520/35-25 Emitter is mounted in a 3.5 mm aluminum housing, as shown below.

 

 

For dichroic beamsplitters and other unmounted filters in a catalog configuration, the size of the filter on the Description and Pricing tab is shown as “rectangular dimensions x nominal substrate thickness”.

As shown below, the FF495-Di03-25x36 dichroic beamsplitter of the GFP-3035D filter set has dimensions 25.2 mm x 35.6 mm, has a nominal substrate thickness of 1.1 mm (also commonly listed on the specifications tab as 1.05 mm), and is unmounted.

 

 

 

The details of the physical filter specifications are also listed on the product Specifications  tab (see below). 

 

 

This tab lists filter thickness, whether mounted or unmounted, unmounted substrate thickness, and dimensional tolerances of filter and substrate thicknesses (here shown bordered in green).

 

 

 

 

 

 

 

 

 

 

 

 

The thickness of a custom sized, unmounted circular or rectangular filter refers to the substrate thickness.

Semrock catalog filters are coated on substrates with standard thicknesses 0.5 mm, 1.05 mm, 2.0 mm, 3.0 mm, 3.5 mm, and 6.0 mm; each product family is coated on one specific thickness.  Before purchase be sure to verify the filter thickness and whether that will be suitable for mount and application requirements.

For questions about filter, substrate or housing thickness, please contact Semrock Technical Support.

Flatness / RWE Classification for Dichroic Beamsplitters 

Wavefront distortion can degrade image quality by reducing contrast or compromising resolution. In several microscopy applications, reducing wavefront distortion is critical to achieving the microscopy method.

Semrock offers an extensive and industry-leading range of catalog filters for a variety of applications with specific Flatness/RWE needs. The Semrock Flatness Classifications listed in following Table provide an intuitive approach to selecting products of appropriate flatness for a given application.

In determining suitable flatness need for a given application, the most important parameter is often the diameter of the beam striking the dichroic beamsplitter surface. Table below shows the Semrock product best suited to the application, for maximum diameter values. For more specific information, and for other beam diameter value and microscopy examples, the Semrock Technical Note and White Paper on this topic [1, 2]  provide additional information on RWE, TWE, and microscopy methods, as well as guidance from a system designer’s perspective.

[1]  Choosing Dichroic Beamsplitters with Flatness/RWE Appropriate to the Microscopy Method

[2] Maximizing the Performance of Advanced Microscopes by Controlling Wavefront Error Using Optical Filters

 

Flatness/RWE Classification

Example 
Applications

Nominal Radium of Curvature

Maximum Reflected Beam Diameter, mm

Reflected Wavefront Error at 632.8 nm, PV

Dichroic Family and Example Part Numbers

Super-resolution / TIRF

 

TIRF, PALM, STORM, STED

 

~ 1275 meters

22.5

< 0.2λ

BrightLine® Laser (Di03-R405-t3-)

~ 255 meters

10

< 1λ

BrightLine® Laser (Di03-R405-t1-)

Image-splitting

 

Splitting of emission signal on a pixel based detector

 

~ 1275 meters

37

< 0.2λ

BrightLine® Image-splitting
(FF509-FDi02-t3-)

 

~ 100 meters

10

< 2λ

BrightLine® Image-splitting (FF509-FDi01-)

Laser

Confocal, combining/splitting laser beams

~ 30 meters

2.5

< 6λ

BrightLine® Laser (Di02-R405-)

RazorEdge Dichroic™  (LPD02-488RU-)

LaserMux™ (LM01-503-)

StopLine® Notch Dichroic (NFD01-488-)

Standard Epi-fluorescence

Widefield fluorescence

~ 6 meters

Not Applicable

>> 6λ

BrightLine® (FF495-Di03-)

Optical Density

Optical Density (OD) is a convenient tool to describe the transmission of light through a highly blocking optical filter (when the transmission is extremely small). OD is defined as the negative of the logarithm (base 10) of the transmission, where the transmission varies between 0 and 1.

OD = – log 10 (T)
or
T = 10 -OD

Optical Density Noise Floor:

Through careful optimization of our measurement equipment and methods, Semrock is able to provide some of the best wide bandwidth Optical Density (OD) measurements possible. Below are typical OD noise floor limitations for most of the measurement data shown on this website.

  • For wavelengths between 320 and 1120 nm, transmission values near or below 3e-7 (Optical Density 6.5) are measurement noise limited.
  • For wavelengths < 320 nm and between 1120 - 1500 nm, transmission values near or below 3e-6 (Optical Density 5.5) are measurement noise limited.
  • For wavelengths > 1500 nm, transmission values near or below 1e-5 (Optical Density 5.0) are measurement noise limited.

Also, for some filters and/or some blocking wavelength ranges, measurements with a noise floor of only about OD 4 are shown.

transmission OD OD rules Optical Density graph

Part Numbering

General product naming conventions are as follows:

GMBW = Guaranteed Minimum Bandwidth

Occasionally there will be an additional ending to the part number after the diameter/dimensions are called out. The addition of a "-D" indicates that a part is unmounted. In this case, the diameter of 25 means that diameter of the glass substrate is 25mm. The addition of a "-N" calls for a part to be unmarked and will not include the Semrock logo, part number, or arrow to indicate the proper orientation.

In the case of a multiband or multi-notch filter, the multiple bands will be called out in an xxx/yyy format, as in this multiband dichroic:

Product naming conventions dual bandpass

 

For those parts who have multiple grades available for purchase, the grade is specified in this way:

product naming conventions with grade

 

 

The following is a guide to help you correlate part numbers and product families. It is not intended for users to create part numbers.

 

AbbreviationStands For...Product Family
BLPBasic Long PassEdgeBasic™
BSPBasic Short PassEdgeBasic™
-Dunhoused, has no metal ringAll
DiDichroicBrightLine®
EE-grade filterRazorEdge®, StopLine®
EmEmissionBrightLine®
FDiFlattest dichroic, used for imaging applicationsBrightLine®
FFFluorescence filterBrightLine®
FRETFluorescence resonance energy transferBrightLine®
HgPeriodic symbol for mercuryMaxLamp®
LDLaser diodeMaxDiode®
LFLaser fluorescenceBrightLine®
LLLaser lineMaxLine®
LMLaser muxLaserMUX
LPLong passBrightLine®, RazorEdge®
LPDLong pass dichroicRazorEdge Dichroic
MEmitterBrightLine®
-Nnot markedAll
NFNotch FilterStopLine®
NIRNear-InfraredNear-IR
PBPPolarizing bandpassPolarizing bandpass
QDQuantum DotBrightLine®
RReflectedBrightLine®, RazorEdge®
SS-grade filterRazorEdge®, StopLine®
SDiShort-pass dichroicBrightLine®
SPShort passRazorEdge®
-STRSutter Threaded RingAll (≤ 2 mm substrate required)
TTransmittedBrightLine®
TBPTunable bandpassVersaChrome®
UU-grade filterRazorEdge®, StopLine®
XExciterBrightLine®

What is Pixel Shift

Pixel shift results when a filter in an imaging path (the emitter and/or dichroic beamsplitter in a fluorescence microscope) deviates the light rays to cause a shift of the image detected on a high-resolution CCD camera.  This shift becomes problematic when two or more images of the same object are acquired using different filter sets and then overlaid in order to simultaneously view fluorescence from multiple fluorophores.   Images produced by different fluorophores (and different filter sets) will not be accurately correlated or combined because each image is shifted by a different amount according to the wedge angles found in each filter set.  To eliminate pixel shift, BrightLine ZEROTM filter sets are manufactured and tested to exacting tolerances to ensure accurate image registration when combining multiple images.

The BrightLine ZERO™ option guarantees that the worst-case image shift when interchanging Semrock ZERO sets will be less than ± 1 pixel, measured relative to the mean image position for a large sample of filter sets.

pixel shift 1

This schematic of a typical epifluorescence geometry (as in a standard microscope) shows how filter wedge causes pixel shift.

pixel shift 2

Polarization

When light is incident on an optical filter at a non-normal angle of incidence, the polarization of the light can be described by two orthogonal vector components associated with the orientation of the electric field of the light wave. The polarization is referenced to the “plane of incidence,” or the plane that is parallel to the normal to the surface of the filter and contains both the incident and the reflected light rays. The polarization component that is perpendicular to the plane of incidence is called the “s” component, and the component that is parallel to the plane of incidence is called the “p” component.

Why might a customer’s filter spectrum measurement differ from the Semrock spectrum?

Commercially available spectrophotometers are optimized for specific scientific and industrial applications. Because their designs have been constrained by price and performance requirements, most commercial spectrophotometers have insufficient precision to accurately measure the spectral characteristics of high-performance optical filters, especially filters with steep spectral edges and deep blocking, which are Semrock hallmarks. Functional limitations in light sources, detectors, and diffraction gratings, and in their combined performance, can create artifacts in the spectral measurement of such high performance optical filters. A customer's filter spectrum measurement may differ from the measurement performed at Semrock due to the presence of these artifacts. Understanding the origins of such measurement discrepancies enables inference of the filter’s actual performance and provides guidance for decision making.


1. Feature Rounding
The spectral features of an optical filter often appear rounded when measured using a conventional spectrophotometer. This artifact occurs when the probe beam is not strictly monochromatic. The slight spectral width of the probe beam causes a "smoothing" effect that rounds sharp transitions. In the graph shown here, feature rounding has softened the transition at the beginning of the transmission band.

2. Noise Floor
A light detector has a sensitivity limit beyond which it cannot report variations in light intensity. This cutoff level sets a limit to the highest optical density (OD, defined as -log10(T)) that a spectrophotometer can measure. The measured OD may, therefore, appear lower than the actual filter performance. When illuminated by light levels below the sensitivity limit, the detection system will report zero signal, but will also report any noise originating within the detector. On a graph this appears as a "noise floor" that corresponds to the highest OD measurable by the spectrophotometer. In the figure, the noise floor is the “noisy” spectral region at the lower left corner of the graph. The noise floor may be wavelength-dependent because of variations in both the light source spectrum and the light detector spectral response.

3. Sideband Artifact “Kink”
A conventional spectrophotometer sometimes reports a "kink" in the spectrum when measuring a filter with an extremely steep transition between blocking and transmission. In the figure, the kink is the point at which the spectrophotometer measurement diverges from the design spectrum. If present, the spectral kink usually appears between OD 2.5 and 4.5, and can give the impression that the filter's spectral transition is less steep than the actual filter performance. The kink occurs because imperfections in the diffraction grating can create noise sidebands in the spatial profile of the probe beam. If a probe beam has significant sideband noise, and if the filter has a very steep edge, then the light from the noise sideband may transmit through the filter's passband even though the filter has blocked the primary portion of the probe beam. The unwanted transmission lowers the measured OD at that wavelength, creating the characteristic “kink” in the sideband.

For further information on the process that Semrock uses to guarantee filter performance, download our white paper, Measurement of Optical Filter Spectra

 

 Measurement Issues

 


 

 

 

 

 

 

 

 

 

 

Example showing design and measured spectra of a Semrock LP03-532RU-25 RazorEdge® filter. The measurement was made using a commercial spectrophotometer

Sutter Threaded Rings

Semrock has worked with Sutter Instruments to design and introduce an integrated filter wheel mounting ring for our optical filters that is threaded to match the openings in a Sutter filter wheel hub. With this inclusive mounting system, the optical filter and threaded retaining ring are incorporated into a single component (see Figure 1). Sutter filter wheels traditionally have been designed to accept optical filters up to 9 mm in thickness with each filter wheel position incorporating a mounting hardware system that consists of a filter cup and retaining ring. The Semrock/Sutter integrated threaded ring solution is designed to be installed after removing the filter cup and retaining ring. By eliminating the need for the cup and ring hardware, a Semrock optical filter and integrated threaded ring mount assembly actually weighs less than the cup and ring it replaces.



Figure 1:
Left: Traditional mounted optical filter. Right: Semrock/Sutter threaded ring mount

 

The Sutter threaded ring option is available for all Semrock “Pinkel” (single-band exciters with multiband emitter) and “Sedat” (single-band exciter and emitters) multicolor filter sets. The option is denoted by the suffix “-S01” appended to the filter set catalog part number and is accompanied by a nominal $20 price premium per excitation filter in “Pinkel” sets, or per excitaiton and emission filter in “Sedat” sets.

Individual catalog filters which have a substrate thickness ≤ 2 mm can be purchased with the Sutter threaded ring option. The option is denoted by the suffix “-STR” appended to the catalog part number and is accompanied by a nominal $20 price premium per filter.

 

Sutter threaded rings are laser marked with an arrow to indicate the Direction of Light Propagation (DLP).

• Excitation filters: the arrow points toward the shoulder, in the direction of the threads. When the filter is threaded into Sutter's Lambda 10 threaded filter wheel used in the excitation light path, the arrow points in the DLP toward the microscope.

• Emission filters: the arrow points away from the shoulder, in the direction of the two notches. When the filter is threaded into Sutter's Lambda 10 threaded filter wheel used in the emission light path, the arrow points in the DLP away from the microscope and towards the detector.

STR Arrow Orientation Exciter
Sutter Threaded Ring Arrow Orientation - Exciter
 STR Arrow Orientation Emitter
Sutter Threaded Ring Arrow Orientation - Emitter

For additional details on installing filters mounted in a Sutter Threaded Ring into a Sutter filter wheel, please read the Sutter Threaded Ring Installation Guide.

Wavenumbers

The "color" of light is generally identified by the distribution of power or intensity as a function of wavelength. Sometimes it is convenient to describe light in terms of "wavenumbers," where the wavenumber (w) is simply equal to the inverse of the wavelength and is therefore proportional to frequency.

Wavenumbers are often used in Raman spectroscopy since the separation of a particular Raman line from the laser line is fixed by the molecular properties of the material and independent of which laser wavelength is used to excite the line. This means that the shift is of constant frequency, regardless of excitation wavelength and can be conveniently expressed in terms of wavenumbers.

wavenumber formula

 

To learn more, please review our technical note Measuring Light with Wavelengths and Wavenumbers.

Wavenumbers

FAQ questions Angle tuning VersaChrome MyLight AOI angle of incidence CHA cone half angle ASCII Bandwidth Clear-aperture CA Custom Coating Filter-thickness Thickness Orientation Arrow Arrow-direction Optical Density Pixel shift ZERO Pixel-Shift Polarization rayleigh Range Substrate thickness Sutter threaded rings Sutter-threaded-rings STR Wavenumbers Flatness Imaging Part Number OD Blocking GMBW FHWM Guaranteed Minimum Full-width-half-max Theoretical-data Measured-data Flatness-of-a-Dichroic-beamsplitter Pixelshift Orientation-of-a-filter Custom-size Sizing Custom-sizing Filter-sizing Effective Index of Refraction neff